Refractometers are used for measuring the refractive index of a material or other physical or chemical properties thereof, that are somehow related to the refractive index. Some of the existing refractometers are based on Bragg diffraction gratings, such as those described in the U.S. Pat. No. 7,184,135 granted on Feb. 27, 2007, and the International Patent Application WO 2006/061543 published on Jun. 15, 2006, the Commissariat a I'Energie Atomique of Paris (France) being the owner thereof. The refractometers therein disclosed use an optical fibre. In the fibre core there is a Bragg diffraction grating, having planes not at right angle with the axis of the fibre. When light propagating through the fibre is incident on the grating a diffracted light is coupled in part to the discrete spectrum of the counter-propagating cladding modes, in part to the continuum of radiative modes sending energy outside the fibre. The diffracted light spreads out in different way between the cladding modes and the radiative modes depending on the refractive index of the medium i.e. the material to be examined in which the fibre is dipped.
According to the U.S. Pat. No. 7,184,135 the measure of the refractive index is obtained by the spectral analysis of the cladding modes to be performed by a very high resolution spectrometer. According to the Patent Application WO 2006/061543, the refractive index is determined by a measure of power which is uncoupled from the fibre by means of the radiative modes. Thus, both methods are based on a physical principle according to which the refractive index influences the value of the effective index of the cladding and radiative modes, and then on how the light diffracted is spread out between such modes. In the methods of the above cited documents an important role in determining the resolution and the measuring range of the instrument is played by values of refractive index of the fibre, which are practically not modifiable, as well as by parameters of the grating. This limits a lot the flexibility of using the described methods.
Further, it should be pointed out that the above described methods achieve high resolutions only with extremely expensive instruments, which are allowable only in optical laboratories. In fact, the method of the above US patent requests a spectrometer having a resolution in the region of a picometer, and the method according to the above International Patent Application needs a sensor and electronics component part, both being able to operate with very unfavourable signal-to-noise ratios.